1. The Field of the Invention
This invention relates to a ship lifting apparatus, or drydock.
2. The Prior Art
Many arrangements have been used and proposed by the prior art for docking vessels or lifting them out of the water for purposes such as repairs and under-body painting and servicing. These arrangements have included a platform between a pair of spaced apart piers and which is first submerged and the vessel floated onto it. The platform and the vessel are then lifted out of the water by raising the platform by means of various hoisting mechanisms mounted on the pier edges. One such drydock raises and lowers the platform carrying the vessel by electrically rotated drum hoists mounted on fixed structures on both sides of the platform and connected to the platform by wire rope cables. A number of such hoists may be employed in a row on each side of the platform, each hoist having an electric driving motor, a cable drum, often a reduction gear and some sort of a brake.
Problems arise with wire cable hoist drydocks because the life of the wire rope cables is short due to the heavy loads carried and the deformation which occurs in travel over drums and pulleys. In addition, the electric driving motors for the drum hoists must be carefully synchronized to provide uniform lift. This may be difficult if the cables stretch appreciably or nonuniformly. The braking system also must be carefully designed to hold the very heavy load safely at any level.
A substantial improvement would be made by providing a ship lifting apparatus which employed lifting mechanisms that do not stretch appreciably, which have long life, and which can be safely and mechanically locked to provide a fail-safe braking system.